1. Field of the Invention
The present invention relates to an improved method for the electrical attachment of a semiconductor die to the leads of a lead frame and the apparatus formed therefrom. More particularly, the present invention relates to the use of multi-layered or laterally-segmented metal/elastomer strips to achieve electrical contact between the bond pads of a semiconductor die and the leads of a lead frame or other conductor pattern in order to eliminate the necessity for wirebonding or direct lead bonding (TAB) to the semiconductor die.
2. Background Art
The most common die-connection technology in the microelectronics industry is wirebonding. As illustrated in FIG. 6, wirebonding generally starts with a semiconductor die 30 bonded by a die-attach adhesive such as a solder or an epoxy to a lead frame paddle or to a discrete substrate 29. A plurality of bond wires 32 are then placed, one at a time, to electrically connect the bond pads 34 to their corresponding leads 36. One end of each bond wire is attached to a bond pad 34 of the semiconductor die 30, and the other bond wire end is attached to a lead 36.
The bond wires 32 are attached through one of three industry standard wirebonding techniques: ultrasonic bondingxe2x80x94using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld, resulting in a so-called wedge-wedge wire bond; thermocompression bondingxe2x80x94using a combination of pressure and elevated temperature to form a weld, resulting in a so-called ball-wedge wire bond; and thermosonic bondingxe2x80x94using a combination of pressure, elevated temperature, and ultrasonic vibration bursts, resulting in a ball-wedge bond similar to that achieved by thermocompression bonding. Although these wirebonding techniques accomplish the goal of forming electrical contact between the semiconductor die 30 (i.e. through the bond pads 34) and each lead 36, all of these techniques have the drawback of requiring very expensive, high-precision, high-speed machinery to attach the individual bond wires 32 between the individual bond pads 34 and the leads 36. Moreover, the preferred bond wire material is gold, which becomes extremely expensive for the vast quantities employed in commercial semiconductor fabrication. Other materials employed in the art, such as silver, aluminum/silicon, aluminum/magnesium, and palladium, while less expensive than gold, still contribute significantly to the cost of achieving die/lead frame electrical connections.
U.S. Pat. No. 4,862,245 issued Aug. 29, 1989 to Pashby et al. illustrates an alternate lead arrangement on the semiconductor die (see FIG. 7). The leads 46 are extended over a semiconductor die 40 (xe2x80x9cleads over chipxe2x80x9d or LOC) toward a central or axial line of bond pads 44 wherein bond wires 42 make the electrical connection between the inner ends of leads 46 and the bond pads 44. Film-type alpha barriers 48 are provided between the semiconductor die 40 and the leads 46, and are adhered to both, thus eliminating the need for a separate die paddle or other die support aside from the leads 46 themselves. The configuration of the ""245 patent assists in limiting the ingress of corrosive environmental contaminants to the active surface of the die, achieves a larger portion of the circuit path length encapsulated in the packaging material applied after wire bonding, and reduces electrical resistance caused by the bond wires 42 by placing the lead ends in closer proximity to the bond pads (i.e., the longer the bond wire, the higher the resistance). Although this configuration offers certain advantages, it still requires that bond wires 42 be individually attached between the bond pads 44 and the leads 46.
U.S. Pat. No. 5,252,853 issued Oct. 12, 1993 to Michii illustrates a configuration similar to U.S. Pat. No. 4,862,245 discussed above. However, the lead is further extended to a position over the bond pad wherein the lead is bonded directly to the bond pad (TAB). Although this direct bonding of the lead to the bond pad eliminates the need for wirebonding, it still requires expensive, highly precise equipment to secure the bond between each lead and its corresponding bond pad.
U.S. Pat. No. 5,140,405 issued Aug. 18, 1992 to King et al. addresses the problem of connecting dies to leads by placing a plurality of semiconductor dies in a housing which is clamped to a plate having conductive pads and leads which are precisely aligned with the terminals of the semiconductor dies. A sheet of anisotropically conductive elastomeric material is interposed between the housing and the plate to make electrical contact. The anisotropically conductive elastomeric material is electrically conductive in a direction across its thickness, but non-conductive across its length and width, such as material generally known as an xe2x80x9celastomeric single axis conductive interconnectxe2x80x9d, or ECPI.
Although the technique of achieving electrical contact between the semiconductor dies and the leads in U.S. Pat. No. 5,140,405 is effective for a plurality of chips, the scheme as taught by the ""405 patent is ill-suited for the production of single chips in commercial quantities. The requirement for a housing and the use of a conductive sheet which covers both the housing surface and the semiconductor dies is simply not cost effective when translated to mass production, single-chip conductor attachment or conductor attachment on less than a substantially wafer scale.
A further industry problem relates to burn-in testing of semiconductor dies. Burn-in is a reliability test of semiconductor dies to identify dies which are demonstrably defective as fabricated, or which would fail prematurely after a short period of proper function. Thus, the die is subjected to an initial heavy duty cycle which elicits latent silicon defects. The typical burn-in process consists of biasing the device against a circuit board or burn-in die, wherein the device is subject to an elevated voltage load while in an oven at temperatures of between about 125-150xc2x0 C. for approximately 24-48 hours.
A burn-in die generally comprises a sheet of polyimide film laminated to copper foil leads with electrolytically plated metal bumps which extend from the surface of the polyimide film through vias to the copper foil leads. However, the industry standard process for electrolytically plating bumps generally results in different circuit intensities to each copper foil lead on the burn-in die due to the use of individual tie bars as electrical paths between a bus bar and the bump ends of the leads disposed in the plating bath. The differences in circuit intensities caused by the variable cross-sections of the tie bars extending to each copper foil lead result in the plated bumps being non-uniform in diameter and height. The differences in bump diameter and height consequently make uniform contact with the terminals on the semiconductor dies to be tested much more difficult. In general, the connection between the semiconductor die and the burn-in die is non-permanent, wherein the semiconductor die is biased with a spring or the like in the burn-in die such that the bond pads on the semiconductor die contact the plated bumps. Thus, even minor variations between the plated bump heights may result in one or more die terminals failing to make contact with one or more plated bumps. This lack of contact will result in a portion of the semiconductor device not being under a voltage load during the burn-in process. Thus, if a latent silicon defect exists in this portion of the semiconductor device, the burn-in process will not be effective and the die cannot be effectively electrically tested in the region where the open circuit exists.
U.S. Pat. No. 5,408,190 issued Apr. 18, 1995 to Wood et al. discloses the use of a Z-axis anisotropic conductive sheet of material to electrically connect the bond pads of a die to an intermediate substrate employed in a burn-in assembly for a bare die. However, it appears that a sheet of the anisotropically conductive material is disposed over the entire die and, in some instances, the anisotropically conductive sheet is used in combination with wire bonds extending from the intermediate substrate to the carrier.
Therefore, it would be advantageous to develop a technique for efficiently attaching dies to leads which eliminates the wirebonding process step or any other equivalent procedure requiring precise alignment of a lead end and bond pad or other die terminal. Further, it would also be advantageous to develop a technique for quickly and efficiently making non-permanent contact between semiconductor dies and burn-in dies which would alleviate the need for close dimensional control of burn-in die contacts and for continuous, precise biased contact of the die under test (DUT) and the burn-in die.
The present invention relates to a novel and unobvious technique for electrical attachment (either permanently or non-permanently) of a semiconductor die to the respective leads of a lead frame or other conductor array, and further relates to a semiconductor die assembly and a burn-in die formed using this technique.
The present invention comprises a semiconductor die, preferably with its respective bond pads in a linear arrangement, and a plurality of leads of a lead frame or other conductor array, which leads extend across the semiconductor die and terminate over (above) their corresponding semiconductor die bond pads. The inner ends of the leads may be of any suitable configuration, including pads which are enhanced with downwardly extending flanges. Electrical connection is made between the leads and their respective bond pads by an elongated strip of anisotropically conductive elastomeric material positioned and compressed between the leads and the semiconductor die. As used herein, the term xe2x80x9canisotropically conductive elastomeric materialxe2x80x9d means and includes a material conductive in a direction transverse to the longitudinal axis or direction of elongation of the strip, but not in the direction of elongation.
The conductive strip is preferably a multi-layer laminate consisting of alternating parallel sheets of a conductive foil and an insulating elastomer, wherein the laminate layers are oriented perpendicular to the planes of both the bond pad and the lead. Thus, the conductive strip is electrically conductive in a direction across its thickness and width (i.e. between the lead and bond pad) but non-conductive across its length (i.e., insulated from electric cross-over between adjacent bond pads or leads). The conductive foil may be any suitable electrically conductive material, such as gold, copper, gold/copper, silver, aluminum, or the like. The insulating elastomer can be any material with insulative properties sufficient to prevent electron flow between the separated, parallel sheets of the conductive foil. The elastomer must be capable of maintaining its resiliency over all anticipated temperature ranges to be encountered by the assembly. A variety of elastomeric compounds as known in the art are suitable.
The number of laminated conductive foils per unit length of the strip, or foil density, must be high enough to form at least one electrically conductive path across each lead/bond pad connection. Preferably, the density of the conductive foils form two or more conductive paths so as to ensure that at least one conductive foil is achieving electrical communication across the lead/bond pad connection.
It is, of course, understood that other available materials having equivalent directional-specific conductive properties can be utilized in place of the conductive strip described, such as material previously referenced and generally known as an xe2x80x9celastomeric single axis conductive interconnectxe2x80x9d, or ECPI.
In a further aspect of the invention, a dielectric or insulative tape is positioned as an alpha barrier between the leads and the semiconductor die to prevent false electronic gate activations or deactivations due to residual impurities in the encapsulation material employed to package the die after electrical connection of the leads, and to insulate the active or main surface of the die from the leads. The insulative tape is attached to the semiconductor die and to the leads with appropriate adhesive layers as known in the art. Preferably, the insulative tape has properties which are conducive to the semiconductor environment. Thus, the polymeric film preferably has a melting temperature in excess of 175xc2x0 C. and does not contain ionizable contaminants such as halides and active metals including sodium, potassium and phosphorus. Polyimide films, such as duPont Kapton(trademark), possess the appropriate properties and can be used as an effective alpha barrier insulative tape. The adhesive attachment of the leads to the die through the tape results in precise maintenance of lead position and simultaneous, elastomerically-biased, lead-to-bond pad electrical connection of all leads of a lead frame or other conductor pattern.
A primary advantage of the present invention is the elimination of the necessity for bond wires. The present invention requires no expensive, high-precision, high-speed machinery to attach the bond wires to the individual bond pads and leads. Furthermore, all electrical connections between the leads and the semiconductor die are simultaneously and adhesively made at ambient temperature upon the contact of the conductive strip with the leads and semiconductor die. This substantially reduces the amount of production time required which, in turn, reduces production costs.
The present invention also has further advantages over both wirebonding or directly bonding the lead to the bond pads. Different thermal coefficients of expansion of the different materials employed in the prior art processes such as TAB result in different rates of thermal expansion and contraction for different elements of the semiconductor die conductive paths when power to the semiconductor die is turned on and off. The differences in thermal coefficients of expansion cause pushing and pulling strains on the components of the semiconductor die. These strains can cause the bond wires or TAB bonds to fatigue and break. However, since the contact between the leads and the bond pads of the present invention is substantially elastic, temperature compensation characteristics of the conductive foil-containing elastomer maintain contact between the leads and the bond pads without fatigue. Furthermore, the elastic qualities of the elastomer allow it to effectively conform to different shaped surfaces, such as the bond pads being either protrusions from the die surface or depressions or recesses in a passivating layer.
The present invention is also advantageous for use in burn-in dies. As previously discussed, the standard burn-in die comprises a sheet of polyimide film laminated to copper foil leads with electrolytically plated metal bumps which extend from the surface of the polyimide film through vias to the copper foil leads. However, the electrolytic bump forming process results in the plated bumps being non-uniform in diameter and height. The differences in bump diameter and height makes uniform contact with the terminals on the DUT""s much more difficult.
The present invention solves the contact problem with burn-in dies. When the semiconductor die in a fixture is placed on the burn-in die and biased with a spring or the like, the conductive strip makes non-permanent contact with the bond pads of the semiconductor die. Since the conductive strip is elastic, the DUT makes proper contact with its respective lead. Thus, the use of plated bumps is completely eliminated and, along with it, the problem of non-uniform bump heights. Furthermore, the present invention does not require as high a precision placement of the semiconductor die on the burn-in die. The characteristics of the multi-layer elastomer allow some variation in the orientation of the semiconductor die while still achieving proper electrical contact between the semiconductor die and the burn-in die ends.